Light-emitting semiconductor device using gallium nitride compound semiconductor

ABSTRACT

A barrier layer made of Al x Ga 1−x N (0&lt;x≦0.18) is formed in a light-emitting semiconductor device using gallium nitride compound having a multi quantum-well (MQW) structure. By controlling a composition ratio x of aluminum (Al) or thickness of the barrier layer, luminous intensity of the device is improved. 
     An n-cladding layer made of Al x Ga 1−x N (0&lt;x≦0.06) is formed in a light-emitting semiconductor device using gallium nitride compound. By controlling a composition ratio x of aluminum or thickness of the n-cladding layer, luminous intensity of the device is improved. 
     A p-type layer and an n-type layer are formed in a light-emitting semiconductor device using gallium nitride compound having a double-hetero junction structure. By controlling a ratio of a hole concentration of the p-type layer and an electron concentration of the n-type layer approximates to 1, luminous intensity of the device is improved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting device using galliumnitride compound semiconductor whose luminous efficiency is improved.Especially, the present invention relates to the device which emits theultraviolet ray.

2. Description of the Related Art

A conventional light-emitting device, which have layers made of galliumnitride compound semiconductor laminated on a substrate, is known tohave the following structure. The device has a sapphire substrate, andon the sapphire substrate the following layers are formed sequentially:a buffer layer made of aluminum nitride (AlN); an n-cladding and/or ann-contact layer of high carrier concentration, which is made of asilicon (Si) doped GaN of n-type conduction; an emission layer having amulti quantum-well (MQW) structure, in which a barrier layer made of GaNand a well layer made of InGaN are laminated alternately; a p-claddinglayer made of magnesium (Mg) doped AlGaN of p-type conduction; and ap-contact layer made of magnesium (Mg) doped GaN of p-type conduction.

And a conventional light-emitting device using gallium nitride compoundsemiconductor which emits the ultraviolet ray is known to have anemission layer made of InGaN or AlGaN. The device having an emissionlayer made of InGaN can obtain an ultraviolet ray having a wavelength oflower than 380 nm, which is emitted from band to band, when acomposition ratio of indium (In) is less than 5.5%. The device having anemission layer made of AlGaN can obtain an ultraviolet ray having awavelength of 380 nm, which is emitted by a pair of donor and acceptor,when a composition ratio of aluminum (Al) is about 16% and the emissionlayer is doped with zinc (Zn) and silicon (Si).

However, a problem persists in luminous efficiency. In the conventionallight-emitting devices using gallium nitride compound semiconductor,conditions for emitting light are not always optimized. Therefore,further improvement has been required, as presently appreciated by thepresent inventors.

SUMMARY OF THE INVENTION

An object of the present invention is to improve luminous efficiency ofa light-emitting device using gallium nitride compound semiconductor.

To achieve the above object, a first aspect of the present invention isto obtain a light-emitting device using gallium nitride semiconductorcomprising an emission layer with a multi quantum-well (MQW) structure,in which a barrier layer and a well layer are formed alternately. Thebarrier layer is made of Al_(x)Ga_(1−x)N (0<x≦0.18).

The second aspect of the present invention is to form the well layermade of In_(y)Ga_(1−y)N (0≦y≦0.1).

The third aspect of the present invention is to form the barrier layerto have a thickness from 2 nm to 10 nm.

The fourth aspect of the present invention is to form the barrier layerto have a thickness from 3 nm to 8 nm.

The fifth aspect of the present invention is to design a luminouswavelength in the ultraviolet ray region.

The sixth aspect of the present invention is to obtain a light-emittingdevice using gallium nitride compound semiconductor comprising anemission layer with a multi quantum-well (MQW) structure, in which abarrier layer and a well layer are formed alternately, and an n-layermade of an impurity-doped Al_(Ga) _(1−x)N (0<x≦0.06).

The seventh aspect of the present invention is to form a strainrelaxation layer made of In_(y)Ga_(1−y)N (0.02≦y≦0.04) which is formedunder the n-layer.

The eighth aspect of the present invention is to form the n-layer tohave a thickness from 50 nm to 300 nm.

The ninth aspect of the present invention is to form the n-layer to havea thickness from 150 nm to 250 nm.

The tenth aspect of the present invention is to design a luminouswavelength to be in the ultraviolet ray range.

The eleventh aspect of the present invention is to obtain alight-emitting device using gallium nitride compound semiconductorcomprising an emission layer with a multi quantum-well (MQW) structure,in which a barrier layer and a well layer are formed alternately, ap-layer, and an n-layer. The emission layer is sandwiched by the p-layerand the n-layer, and a ratio of an electron concentration of the n-layerto a hole concentration of the p-layer (electron/hole) is from 0.5 to2.0.

The twelfth aspect of the present invention is to obtain alight-emitting device using gallium nitride compound semiconductorcomprising an emission layer with a multi quantum-well (MQW) structure,in which a barrier layer and a well layer are formed alternately, ap-layer, and an n-layer. The emission layer is sandwiched by the p-layerand the n-layer, and a ratio of an electron concentration of the n-layerto a hole concentration of the p-layer (electron/hole) is from 0.7 to1.43.

The thirteenth aspect of the present invention is to obtain alight-emitting device using gallium nitride compound semiconductorcomprising an emission layer with a multi quantum-well (MQW) structure,in which a barrier layer and a well layer are formed alternately, ap-layer, and an n-layer. The emission layer is sandwiched by the p-layerand the n-layer, and a ratio of an electron concentration of the n-layerto a hole concentration of the p-layer (electron/hole) is from 0.8 to1.25.

The fourteenth aspect of the present invention is to design a luminouswavelength in the ultraviolet ray range.

With respect to a gallium nitride compound semiconductor which satisfiesthe formula Al_(x)Ga_(1−x−y)In_(y)N, the larger a composition ratio x ofaluminum (Al), is, the larger a band gap energy becomes, and the largera composition ratio y of indium (In) is, the smaller the band gap energybecomes. With respect to a light-emission device using gallium nitridecompound semiconductor which has an emission layer with a multiquantum-well (MQW) structure, an energy barrier between a well layer anda barrier layer becomes larger when the barrier layer is made ofAl_(x)Ga_(1−x)N. A luminous intensity of the device is strongly relatedto a composition ratio x of aluminum (Al) in Al_(x)Ga_(1−x)N barrierlayer. Various samples of a barrier layer made of Al_(x)Ga_(1−x)N, eachhaving a different composition ratio x of aluminum (Al), are formed andthe electroluminescence (EL) luminous intensity is measured. FIG. 2illustrates a graph of the electroluminescence (EL) luminous intensity.As shown in FIG. 2, the luminous intensity of the light-emitting devicebecomes larger in accordance with the composition ratio of aluminum(Al). The composition ratio x should be preferably in the range of0.06≦x≦0.18. When x, or a composition ratio of aluminum (Al), is smallerthan 0.06, an effect for mixing aluminum (Al) in the barrier layer issmall. When x is larger than 0.18, a lattice matching of the barrierlayer becomes worse and as a result luminous intensity is lowered.

Samples of a light-emitting device having a well layer made ofIn_(y)Ga_(1−y)N which has a smaller band gap are formed. When y, or acomposition ratio of indium (In), is smaller than 0.1, a crystallizationof the well layer becomes worse, and the device cannot have a largeluminous intensity.

Various samples of a barrier layer each having a different thickness areformed. FIG. 3 illustrates the electroluminescence (EL) luminousintensity of the light-emitting device having the barrier layer made ofAl_(x)Ga_(1−x)N. As shown in FIG. 3, the thickness of the barrier layershould be preferably in the range of 2 nm to 10 nm, more preferably 3 nmto 8 nm.

When an n-cladding layer which contacts to the emission layer is made ofAl_(x)Ga_(1−x)N (0≦x≦0.06), holes in the emission layer is preventedfrom leaking to the lower n-layer side through the n-cladding layer.Also, a lattice mismatching of the emission layer which is grown on then-cladding layer can be relaxed and as a result a crystallization of theemission layer is improved. Accordingly, a luminous efficiency of thelight-emitting device can be improved.

A luminous intensity of the light-emitting device is strongly related toa composition ratio x of aluminum (Al) in Al_(x)Ga_(1−x)N n-claddinglayer. Various samples of an n-cladding layer made of Al_(x)Ga_(1−x)N,each having a different composition ratio x of aluminum (Al), are formedand the electroluminescence (EL) luminous intensity is measured. FIG. 5illustrates a graph of the electroluminescence (EL) luminous intensity.As shown in FIG. 5, the luminous intensity of the light-emitting devicebecomes larger in accordance with the composition ratio of aluminum(Al). And when the composition ratio x is around 0.05, luminousintensity of the device shows its peak. The composition ratio x shouldbe preferably in the range of 0.03≦x≦0.06. When x, or a compositionratio of aluminum (Al), is smaller than 0.03, the device becomes justlike a device without an n-cladding layer and holes leak to the lowern-layer side through the n-cladding layer. When x is larger than 0.06, acrystallization of the emission layer is lowered because of too muchaluminum (Al) existing in the n-cladding layer, and as a result theluminous intensity of the device is lowered. various samples of ann-cladding layer each having a different thickness are formed. FIG. 6illustrates the electroluminescence (EL) luminous intensity of thelight-emitting device having the n-cladding layer made ofAl_(x)Ga_(1−x)N. As shown in FIG. 6, the luminous intensity of thedevice shows its peak when the thickness of the n-cladding layer isaround 200 nm. The thickness of the n-cladding layer should bepreferably in the range of 50 nm to 300 nm, more preferably 150 nm to250 nm.

With respect to a light-emitting device using gallium nitride compoundsemiconductor which has a double-hetero junction structure, forming ann-type layer is easier than forming a p-type layer. A hole concentrationof the p-type layer is smaller than an electron concentration of then-type layer. FIGS. 8 and 9 illustrate graphs of the electroluminescence(EL) luminous intensity of the light-emitting device when each electronconcentrations of an n-cladding layer and an n-contact layer is variedin order that a ratio of the electron concentration of each n-typelayers, the n-cladding layer and the n-contact layer to a holeconcentration of each p-type layers, a p-cladding layer and a p-contactlayer, respectively, approximates to 1. Here a hole concentration of thep-cladding layer and the p-contact layer is 2×10¹⁷/cm³ and 7×10¹⁷/cm³,respectively.

A luminous intensity of the light-emitting device is strongly related toan electron concentration of Al_(0.05)Ga_(0.95)N n-cladding layer.Various samples of an n-cladding layer made of Al_(0.05)Ga_(0.95)N, eachhaving a different electron concentration, are formed and theelectroluminescence (EL) luminous intensity is measured. FIG. 8illustrates a graph of the electroluminescence (EL) luminous intensity.As shown in FIG. 8, the luminous intensity of the light-emitting deviceshows its peak when the electron concentration of the n-cladding layeris around 8×10¹⁷/cm³.

Also, the luminous intensity of the light-emitting device is stronglyrelated to an electron concentration of GaN n-contact layer. Varioussamples of an n-contact layer made of GaN, each having a differentelectron concentration, are formed and the electroluminescence (EL)luminous intensity is measured. FIG. 9 illustrates a graph of theelectroluminescence (EL) luminous intensity. As shown in FIG. 9, theluminous intensity of the light-emitting device becomes larger inaccordance that the electron concentration of GaN n-cladding layerbecomes 1.1×10¹⁸/cm³, 8×10¹⁷/cm³, and 4×10¹⁷/cm³. This proves that arecombination of electrons and holes occurs at the inside of theemission layer. In short, when an electron concentration of then-cladding layer or the n-contact layer is larger than a holeconcentration of the p-cladding layer or the p-contact layer, electronstend to recombine with holes at the p-contact or the p-cladding layerside from the emission layer. And if a recombination of electrons andholes which does not emit lights increases under this condition, it isconsidered that balancing a hole concentration of the p-cladding or thep-contacting layer and an electron concentration of the n-cladding orthe n-contact layer is effective for decreasing the non-emissiverecombination of electrons and holes.

Here the n-cladding layer made of GaN needs to have an electronconcentration of at least 1×10¹⁷/cm³ in order to form an electrode,inject electrons and drive the light-emitting device.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, and characteristics of the present inventionwill become apparent upon consideration of the following description andthe appended claims with reference to the accompanying drawings, all ofwhich form a part of the specification, and wherein reference numeralsdesignate corresponding parts in the various figures, wherein:

FIG. 1 is a sectional view of a light-emitting device 100 using galliumnitride compound in accordance with the first embodiment of the presentinvention;

FIG. 2 is a graph showing the correlation of a composition ratio x ofaluminum (Al) and a luminous intensity in the barrier layer 151 made ofAl_(x)Ga_(1−x)N, in accordance with the first embodiment of the presentinvention;

FIG. 3 is a graph showing the correlation of a thickness and a luminousintensity of the barrier layer 151 made of Al_(x)Ga_(1−x)N in accordancewith the first embodiment of the present invention;

FIG. 4 is a sectional view of a light-emitting device 200 using galliumnitride compound in accordance with the second embodiment of the presentinvention;

FIG. 5 is a graph showing the correlation of a composition ratio x ofaluminum (Al) and a luminous intensity in the n-cladding layer 214B madeof Al_(x)Ga_(1−x)N, in accordance with the second embodiment of thepresent invention;

FIG. 6 is a graph showing the correlation of a thickness and a luminousintensity of the n-cladding layer 214B made of Al_(x)Ga_(1−x)N, inaccordance with the second embodiment of the present invention;

FIG. 7 is a sectional view of a light-emitting device 300 using galliumnitride compound in accordance with the third embodiment of the presentinvention;

FIG. 8 is a graph showing the correlation of an electron concentrationand a luminous intensity of the n-cladding layer in accordance with thethird embodiment of the present invention; and

FIG. 9 is a graph showing the correlation of an electron concentrationand a luminous intensity of the n-contact layer in accordance with thethird embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described hereinbelow with reference tospecific embodiments.

(First Embodiment)

FIG. 1 illustrates a sectional view of a light-emitting device 100 usinggallium nitride (GaN) compound semiconductor formed on a sapphiresubstrate 111. The light-emitting device 100 has a sapphire substrate111 which has a buffer layer 112 made of nitride aluminum (AlN) having athickness of 25 nm and an n-cladding or an n-contact layer (n⁺-layer)113 made of silicon (Si) doped GaN and having a thickness of 3000 nmsuccessively thereon.

And a strain relaxation layer 114 made of a non-dopedIn_(0.03)Ga_(0.97)N having a thickness of about 180 nm is formed on then-cladding layer or the n-contact layer (n⁺-layer) 113. The strainrelaxation layer 114 functions to relax a stress to an emission layer115, generated by the difference between thermal expansion coefficientsof the sapphire substrate 111 and the emission layer 115.

An emission layer 115 is constructed with a multi quantum-well (MQW)structure, which is made of six barrier layers 151 made ofAl_(0.13)Ga_(0.87)N having a thickness of about 3.5 nm and five welllayers 152 made of In_(0.05)Ga_(0.95)N having a thickness of about 3 nmlaminated alternately, is formed on the strain relaxation layer 114. Ap-cladding layer 116 made of a p-type Al_(0.15)Ga_(0.85)N having athickness of about 25 nm is formed on the emission layer 115. Further, ap-contact layer 117 made of a p-type GaN having a thickness about 100 nmis formed on the p-cladding layer 116.

An electrode 118A which transmits lights is formed by a metal deposit onthe p-contact layer 117 and an electrode 118B is formed on the n⁺-layer113. The electrode 118A which transmits lights is constructed with about1.5 nm in thickness of cobalt (Co), which contacts to the p-contactlayer 117, and about 6 nm in thickness of gold (Au), which contacts tothe cobalt (Co). The electrode 118B is constructed with about 20 nm inthickness of vanadium (V) and about 1800 nm in thickness of aluminum(Al) or an alloy including aluminum (Al). And an electrode pad 120having a thickness about 1500 nm is formed on the electrode 118A. Theelectrode pad 120 is made of cobalt (Co), nickel (Ni) or vanadium (V),and gold (Au) or aluminum (Al), or an alloy including at least one ofthese metals.

Then a method for manufacturing the light-emitting device 100 isexplained hereinafter.

Each of the layers of the light-emitting device 100 is formed by gaseousphase epitaxial growth, called metal organic vapor phase deposition(hereinafter MOVPE). The gases employed in this process were ammonia(NH₃), a carrier gas (H₂ or N₂), trimethyl gallium (Ga(CH₃)₃)(hereinafter TMG), trimethyl aluminum (Al(CH₃)₃) (hereinafter TMA),trimethyl indium (In(CH₃)₃) (hereinafter TMI), silane (SiH₄), andbiscyclopentadienyl magnesium (Mg(C₅H₅)₂) (hereinafter CP₂Mg).

The single crystalline sapphire substrate 111 was placed on a susceptorin a reaction chamber for the MOVPE treatment after its main surface ‘a’was cleaned by an organic washing solvent and heat treatment. Then thesapphire substrate 111 was baked at 1100° C. by H₂ vapor fed into thechamber under normal pressure.

About 25 nm in thickness of AlN buffer layer 112 was formed on thesurface ‘a’, of the baked sapphire substrate 111 under conditionscontrolled by lowering the temperature in the chamber to 400° C.,keeping the temperature constant, and concurrently supplying H₂, NH₃ andTMA.

About 3 μm in thickness of GaN was formed on the buffer layer 112, as ann-cladding or n-contact layer (n⁺-layer) 113 with an electronconcentration of 2×10¹⁸/cm³, under conditions controlled by keeping thetemperature of the sapphire substrate 111 at 1150° C. and concurrentlysupplying H₂, NH₃, TMG and silane.

About 180 nm in thickness of non-doped In_(0.03)Ga_(0.97)N was formed onthe n⁺-layer 113, as a strain relaxation layer 114, under conditionscontrolled by lowering the temperature of the sapphire substrate 111 to850° C., keeping the temperature constant and concurrently supplying N₂or H₂, NH₃, TMG and TMI.

Then a barrier layer 151 made of Al_(0.33)Ga_(0.87)N was formed underconditions controlled by raising the temperature of the sapphiresubstrate 111 to 1150° C. again, keeping the temperature constant andconcurrently supplying N₂ or H₂, NH₃, TMG and TMA. And about 3 nm inthickness of In_(0.05)Ga_(0.95)N was formed on the barrier layer 151, asa well layer 152, concurrently supplying N₂ or H₂, NH₃, TMG and TMI.Similarly, four pairs of the barrier layer 151 and the well layer 152were formed in sequence under the same respective conditions, and then abarrier layer 151 made of Al_(x)Ga_(1−x)N was formed on the fifth pairof the barrier layer 151 and the well layer 152. Accordingly, anemission layer 115 having a multi-quantum well (MQW) structure wasformed.

About 25 nm in thickness of Mg-doped p-type Al_(0.15)Ga_(0.85)N wasformed on the emission layer 115, as a p-cladding layer 116, underconditions controlled by keeping the temperature of the sapphiresubstrate 111 at 1150° C. and concurrently supplying N₂ or H₂, NH₃, TMG,TMA and CP₂Mg.

About 100 nm in thickness of Mg-doped p-type GaN was formed on thep-cladding layer 116, as a p-contact layer 117, under conditionscontrolled by keeping the temperature of the sapphire substrate 111 at1100° C. and concurrently supplying N₂ or H₂, NH₃, TMG, and CP₂Mg.

An etching mask is formed on the p-contact layer 117, and apredetermined region of the mask is removed. Then, exposed portions ofthe p-contact layer 117, the p-cladding layer 116, the emission layer115, the strain relaxation layer 114, and some part of the n⁺-layer 113were etched by a reactive ion etching using gas including chlorine (Cl).Accordingly, the surface of the n⁺-layer 113 was exposed.

Then, an electrode 118B and an electrode 118A which transmits lightswere formed on the n⁺-layer 113 and the p-contact layer 117,respectively, as follows.

(1) A photoresist layer was laminated on the n⁺-layer 113. A window wasformed on a fixed region of the exposed surface of the n⁺-layer 113 bypatterning using photolithography. After exhausting in high vacuum lowerthan 10⁻⁴ Pa vacuum order, about 20 nm in thickness of vanadium (V) andabout 1800 nm in thickness of aluminum (Al) were deposited on thewindow. Then, the photoresist layer laminated on the n⁺-layer 113 wasremoved. Accordingly, the electrode 118B was formed on the exposedsurface of the n⁺-layer 113.

(2) A photoresist layer was laminated on the p-contact layer 117. Thephotoresist layer of an electrode forming part on the p-contact layer117 was removed by patterning using photolithography, and a window wasformed there.

(3) After exhausting in high vacuum lower than 10⁻⁶ Torr vacuum order,about 1.5 nm in thickness of cobalt (Co) and about 6 nm in thickness ofgold (Au) were formed in sequence on the photoresist layer and theexposed surface of the p-contact layer 117 in a reaction chamber fordeposit.

(4) The sample was took out from the reaction chamber for deposit. Thencobalt (Co) and gold (Au) laminated on the photoresist layer wereremoved by a lift-off, and an electrode 118A which transmits lights isformed on the p-contact layer 117.

(5) To form an electrode pad 120 for a bonding, a window was formed on aphotoresist layer, which was laminated uniformly on the electrode 118A.About 1.5 μm in thickness of cobalt (Co), nickel (Ni) or vanadium (V)and gold (Au), aluminum (Al) or an alloy including at least one of thosemetals were deposited on the photoresist layer. Then, as in the process(4), cobalt (Co), nickel (Ni) or vanadium (V) and gold (Au), aluminum(Al) or an alloy including at least one of those metals laminated on thephotoresist layer were removed by a lift-off, and an electrode pad 120was formed.

(6) After the atmosphere of the sample was exhausted by a vacuum pump,O₂ gas was supplied until the pressure becomes 3 Pa. Under conditionscontrolled by keeping the pressure constant and keeping the temperatureof the atmosphere about 550° C., the sample was heated for about 3 min.Accordingly, the p-contact layer 117 and the p-cladding layer 116 werechanged to have lower resistive p-type, and the p-contact layer 117 andthe electrode 118A, and the n⁺-layer 113 and the electrode 118B,respectively, are alloyed.

Through the process of (1) to (6), the light-emitting device 100 wasformed.

Various samples of a barrier layer made of Al_(x)Ga_(1−x)N, each havinga different composition ratio x of aluminum (Al), were formed in thesame process described above. FIG. 2 illustrates the electroluminescence(EL) luminous intensity of the light-emitting device 100 having thebarrier layer made of Al_(x)Ga_(1−x)N. As shown in FIG. 2, the luminousintensity of the light-emitting device 100 becomes larger in accordancewith the composition ratio of aluminum (Al). The composition ratio xshould be preferably in the range of 0.06≦x≦0.18, more preferably0.1≦x≦0.14.

Various samples of a barrier layer each having a different thicknesswere formed. FIG. 3 illustrates the electroluminescence (EL) luminousintensity of the light-emitting device 100 having the barrier layer madeof Al_(x)Ga_(1−x)N. As shown in FIG. 3, the thickness of the barrierlayer should be preferably in the range of 2 nm to 10 nm, morepreferably 3 nm to 8 nm.

In the first embodiment, the light-emitting device 100 having the strainrelaxation layer 114 is shown. Alternatively, the layer should not belimited to a strain relaxation layer. Alternatively, an n-cladding layercan be formed in place of the strain relaxation layer 114.

Alternatively, the well layer, the p-cladding layer, the n-contact layerand the p-contact layer, or all the layers formed in the light-emittingdevice 100 except the barrier layer, can be made of quaternary, ternary,or binary nitride compound semiconductor which satisfies the formulaAl_(x)Ga_(1−x)N, (0≦x≦1, 0≦y≦1), having an arbitrary composition ratio.Also, the strain relaxation layer can be made of In_(x)Ga_(1−x)N(0<x<l), having an arbitrary composition ratio.

The luminous efficiency of the light-emitting device 100 may becomesmaller without a strain relaxation layer, but it can be larger thanthat of the conventional light-emitting device.

In the first embodiment, magnesium (Mg) was used as a p-type impurity.Alternatively, Group II elements such as beryllium (Be), zinc (Zn), etc.can be used.

The device in the present invention can be applied not only to alight-emitting device but also a light-receiving device.

(Second Embodiment)

FIG. 4 illustrates a sectional view of a light-emitting device 200 usinggallium nitride (GaN) compound semiconductor formed on a sapphiresubstrate 211. The light-emitting device 200 has a sapphire substrate211 which has a buffer layer 212 made of nitride aluminum (AlN) having athickness of 25 nm and an n-contact layer 213 made of silicon (Si) dopedGaN and having a thickness of 3 μm successively thereon.

And a strain relaxation layer 214A made of a non-dopedIn_(0.03)Ga_(0.97)N having a thickness of about 180 nm is formed on then-contact layer 213. The strain relaxation layer 214A functions to relaxa stress to an emission layer 215, generated by the difference betweenthermal expansion coefficients of the sapphire substrate 211 and theemission layer 215. And an n-cladding layer 214B made of a silicon (Si)doped Al_(0.05)Ga_(0.95)N having a thickness of about 200 nm is formedon the strain relaxation layer 214A.

An emission layer 215 is constructed with a multi quantum-well (MQW)structure, which is made of six barrier layers 251 made ofAl_(0.13)Ga_(0.87)N having a thickness of about 3.5 nm and five welllayers 252 made of In_(0.05)Ga_(0.95)N having a thickness of about 3 nmlaminated alternately, is formed on the n-cladding layer 214B. Ap-cladding layer 216 made of a p-type Al_(0.15)Ga_(0.85)N having athickness of about 25 nm is formed on the emission layer 215. Further, ap-contact layer 217 made of a p-type GaN having a thickness about 100 nmis formed on the p-cladding layer 216.

An electrode 218A which transmits lights is formed by a metal deposit onthe p-contact layer 217 and an electrode 218B is formed on the n-contactlayer 213. The electrode 218A which transmits lights is constructed withabout 1.5 nm in thickness of cobalt (Co), which contacts to thep-contact layer 217, and about 6 nm in thickness of gold (Au), whichcontacts to the cobalt (Co). The electrode 218B is constructed withabout 20 nm in thickness of vanadium (V) and about 1800 nm in thicknessof aluminum (Al) or an alloy including aluminum (Al). And an electrodepad 220 having a thickness about 1500 nm is formed on the electrode218A. The electrode pad 220 is made of cobalt (Co), nickel (Ni) orvanadium (V), and gold (Au) or aluminum (Al), or an alloy including atleast one of these metals.

Then a method for manufacturing the light-emitting device 200 isexplained hereinafter.

Each of the layers of the light-emitting device 200 is formed by gaseousphase epitaxial growth, called metal organic vapor phase deposition(hereinafter MOVPE). The gases employed in this process were ammonia(NH₃), a carrier gas (H₂ or N₂), trimethyl gallium (Ga(CH₃)₃)(hereinafter TMG), trimethyl aluminum (Al(CH₃)₃) (hereinafter TMA),trimethyl indium (In(CH₃)₃) (hereinafter TMI), silane (SiH₄), andbiscyclopentadienyl magnesium (Mg(C₅H₅)₂) (hereinafter CP₂Mg).

The single crystalline sapphire substrate 211 was placed on a susceptorin a reaction chamber for the MOVPE treatment after its main surface ‘a’was cleaned by an organic. washing solvent and heat treatment. Then thesapphire substrate 211 was baked at 1100° C. by H₂ vapor fed into thechamber under normal pressure.

About 25 nm in thickness of AlN buffer layer 212 was formed on thesurface ‘a’ of the baked sapphire substrate 211 under conditionscontrolled by lowering the temperature in the chamber to 400° C.,keeping the temperature constant, and concurrently supplying H₂, NH₃ andTMA.

About 3 μm in thickness of GaN was formed on the buffer layer 212, as ann-contact layer 213 with an electron concentration of 2×10¹⁸/cm³, underconditions controlled by keeping the temperature of the sapphiresubstrate 211 at 1150° C. and concurrently supplying H₂, NH₃, TMG andsilane.

About 180 nm in thickness of non-doped In_(0.03)Ga_(0.97)N was formed onthe n-contact layer 213, as a strain relaxation layer 214A, underconditions controlled by lowering the temperature of the sapphiresubstrate 211 to 850° C., keeping the temperature constant andconcurrently supplying N₂ or H₂. NH₃, TMG and TMI.

After forming the strain relaxation layer 214A, about 200 nm inthickness of Al_(0.95)Ga_(0.95)N was formed on the strain relaxationlayer 214A, as an n-cladding layer 214B with an electron concentrationof 2×10¹⁷/cm³, under conditions controlled by raising the temperature ofthe sapphire substrate 211 to 1150° C., keeping the temperature constantand concurrently supplying N₂ or H₂, NH₃, TMG, TMA and silane.

About 3.5 nm in thickness of Al_(0.13)Ga_(0.87)N was formed on then-cladding layer 214B, as a barrier layer 251, concurrently supplying N₂or H₂, NH₃, TMG and TMA. And about 3 nm in thickness ofIn_(0.05)Ga_(0.95)N was formed on the barrier layer 215, as a well layer252, concurrently supplying N₂ or H₂, NH₃, TMG and TMI. Similarly, fourpairs of the barrier layer 251 and the well layer 252 were formed insequence under the same respective conditions, and then a barrier layer251 made of Al_(0.13)Ga_(0.87)N was formed on the fifth pair of thebarrier layer 251 and the well layer 252. Accordingly, an emission layer215 having a multi-quantum well (MQW) structure was formed.

About 25 nm in thickness of Mg-doped p-type Al_(0.15)Ga_(0.85)N wasformed on the emission layer 215, as a p-cladding layer 216, underconditions controlled by keeping the temperature of the sapphiresubstrate 111 at 1150° C. and concurrently supplying N₂ or H₂, NH₃, TMG,TMA and CP₂Mg.

About 100 nm in thickness of Mg-doped p-type GaN was formed on thep-cladding layer 216, as a p-contact layer 217, under conditionscontrolled by keeping the temperature of the sapphire substrate 211 at1100° C. and concurrently supplying N₂ or H₂, NH₃, TMG, and CP₂Mg.

An etching mask is formed on the p-contact layer 217, and apredetermined region of the mask is removed. Then, exposed portions ofthe p-contact layer 217, the p-cladding layer 216, the emission layer215, the strain relaxation layer 214A, and some part of the n-contactlayer 213 were etched by a reactive ion etching using gas includingchlorine (Cl). Accordingly, the surface of the n-contact layer 213 wasexposed.

Then, an electrode 218B and an electrode 218A which transmits lightswere formed on the n-contact layer 213 and the p-contact layer 217,respectively, as follows.

(1) A photoresist layer was laminated on the n-contact layer 213. Awindow was formed on a fixed region of the exposed surface of then-contact layer 213 by patterning using photolithography. Afterexhausting in high vacuum lower than 10⁻⁴ Pa vacuum order, about 20 nmin thickness of vanadium (V) and about 1800 nm in thickness of aluminum(Al) were deposited on the window. Then, the photoresist layer laminatedon the n-contact layer 213 was removed. Accordingly, the electrode 218Bwas formed on the exposed surface of the n-contact layer 213.

(2) A photoresist layer was laminated on the p-contact layer 217. Thephotoresist layer of an electrode forming part on the p-contact layer217 was removed by patterning using photolithography, and a window wasformed there.

(3) After exhausting in high vacuum lower than 10⁻⁶ Torr vacuum order,about 1.5 nm in thickness of cobalt (Co) and about 6 nm in thickness ofgold (Au) were formed in sequence on the photoresist layer and theexposed surface of the p-contact layer 217 in a reaction chamber fordeposit.

(4) The sample was took out from the reaction chamber for deposit. Thencobalt (Co) and gold (Au) laminated on the photoresist layer wereremoved by a lift-off, and an electrode 218A which transmits lights isformed on the p-contact layer 217.

(5) To form an electrode pad 220 for a bonding, a window was formed on aphotoresist layer, which was laminated uniformly on the electrode 218A.About 1500 nm in thickness of cobalt (Co), nickel (Ni) or vanadium (V)and gold (Au), aluminum (Al) or an alloy including at least one of thosemetals were deposited on the photoresist layer. Then, as in the process(4), cobalt (Co), nickel (Ni) or vanadium (V) and gold (Au), aluminum(Al) or an alloy including at least one of those metals laminated on thephotoresist layer were removed by a lift-off, and an electrode pad 220was formed.

(6) After the atmosphere of the sample was exhausted by a vacuum pump,O₂ gas was supplied until the pressure becomes 3 Pa. Under conditionscontrolled by keeping the pressure constant and keeping the temperatureof the atmosphere about 550° C., the sample was heated for about 3 min.Accordingly, the p-contact layer 217 and the p-cladding layer 216 werechanged to have lower resistive p-type, and the p-contact layer 217 andthe electrode 218A, and the n-contact layer 213 and the electrode 218B,respectively, are alloyed.

Through the process of (1) to (6), the light-emitting device 200 wasformed.

Various samples of an n-cladding layer made of Al_(x)Ga_(1−x)N, eachhaving a different composition ratio x of aluminum (Al), were formed inthe same process described above. FIG. 5 illustrates theelectroluminescence (EL) luminous intensity of the light-emitting device200 having the n-cladding layer made of Al_(x)Ga_(1−x)N. As shown inFIG. 5, the luminous intensity of the light-emitting device 200 becomeslarger in accordance with the composition ratio of aluminum (Al). Thecomposition ratio x should be preferably in the range of 0.03≦x≦0.06,more preferably 0.04≦x≦0.055.

Various samples of an n-cladding layer each having a different thicknesswere formed. FIG. 6 illustrates the electroluminescence (EL) luminousintensity of the light-emitting device 100 having the n-cladding layermade of Al_(x)Ga_(1−x)N. As shown in FIG. 6, the thickness of thebarrier layer should be preferably in the range of 50 nm to 300 nm, morepreferably 150 nm to 250 nm.

In the second embodiment, the light-emitting device 200 has the emissionlayer 215 with multi quantum-well (MQW) structure. Alternatively, theemission layer 215 can have a single quantum-well structure.

Alternatively, the barrier layer, the well-layer, the p-cladding layer,the n-contact layer and the p-contact layer, or all the layers formed inthe light-emitting device 200 except the n-cladding layer and the strainrelaxation layer, can be made of quaternary, ternary, or binary nitridecompound semiconductor which satisfies the formulaAl_(x)Ga_(1−x−y)In_(y)N, (0≦x≦1, 0≦y≦1), having an arbitrary compositionratio. Also, the strain relaxation layer can be made of In_(x)Ga_(1−x)N(0<x<1), having an arbitrary composition ratio.

The luminous efficiency of the light-emitting device 200 may becomesmaller without a strain relaxation layer, but it can be larger thanthat of the conventional light-emitting device.

In the second embodiment, magnesium (Mg) was used as a p-type impurity.Alternatively, Group II elements such as beryllium (Be), zinc (Zn), etc.can be used.

The device in the present invention can be applied not only to alight-emitting device but also a light-receiving device.

(Third Embodiment)

FIG. 7 illustrates a sectional view of a light-emitting device 300 usinggallium nitride (GaN) compound semiconductor formed on a sapphiresubstrate 311. The light-emitting device 300 has a sapphire substrate311 which has a buffer layer 312 made of nitride aluminum (AlN) having athickness of 25 nm and an n-contact layer 313 made of silicon (Si) dopedGaN and having a thickness of 3 μm successively thereon.

And a strain relaxation layer 314A made of a non-dopedIn_(0.03)Ga_(0.97)N having a thickness of about 180 nm is formed on then-contact layer 313. The strain relaxation layer 314A functions to relaxa stress to an emission layer 315, generated by the difference betweenthermal expansion coefficients of the sapphire substrate 311 and theemission layer 315. And an n-cladding layer 314B made of a silicon (Si)doped Al_(0.05)Ga_(0.95)N having a thickness of about 200 nm is formedon the strain relaxation layer 314A.

An emission layer 315 is constructed with a multi quantum-well (MQW)structure, which is made of six barrier layers 351 made ofAl_(0.13)Ga_(0.87)N having a thickness of about 3.5 nm and five welllayers 352 made of In_(0.05)Ga_(0.95)N having a thickness of about 3 nmlaminated alternately, is formed on the n-cladding layer 314B. Ap-cladding layer 316 made of a p-type Al_(0.5)Ga_(0.85)N having athickness of about 25 nm is formed on the emission layer 315. Further, ap-contact layer 317 made of a p-type GaN having a thickness about 100 nmis formed on the p-cladding layer 316.

An electrode 318A which transmits lights is formed by a metal deposit onthe p-contact layer 317 and an electrode 318B is formed on the n-contactlayer 313. The electrode 318A which transmits lights is constructed withabout 1.5 nm in thickness of cobalt (Co), which contacts to thep-contact layer 317, and about 6 nm in thickness of gold (Au), whichcontacts to the cobalt (Co). The electrode 318B is constructed withabout 20 nm in thickness of vanadium (V) and about 1800 nm in thicknessof aluminum (Al) or an alloy including aluminum (Al). And an electrodepad 320 having a thickness about 1500 nm is formed on the electrode318A. The electrode pad 320 is made of cobalt (Co), nickel (Ni) orvanadium (V), and gold (Au) or aluminum (Al), or an alloy includingthese metals.

Then a method for manufacturing the light-emitting device 300 isexplained hereinafter.

Each of the layers of the light-emitting device 300 is formed by gaseousphase epitaxial growth, called metal organic vapor phase deposition(hereinafter MOVPE). The gases employed in this process were ammonia(NH₃), a carrier gas (H₂ or N₂), trimethyl gallium (Ga(CH₃)₃)(hereinafter TMG), trimethyl aluminum (Al(CH₃)₃) (hereinafter TMA),trimethyl indium (In(CH₃)₃) (hereinafter TMI), silane (SiH₄), andbiscyclopentadienyl magnesium (Mg(C₅H₅)₂) (hereinafter CP₂Mg).

The single crystalline sapphire substrate 311 was placed on a susceptorin a reaction chamber for the MOVPE treatment after its main surface ‘a’was cleaned by an organic washing solvent and heat treatment. Then thesapphire substrate 311 was baked at 1100° C. by H₂ vapor fed into thechamber under normal pressure.

About 25 nm in thickness of AlN buffer layer 312 was formed on thesurface ‘a’, of the baked sapphire substrate 311 under conditionscontrolled by lowering the temperature in the chamber to 400° C.,keeping the temperature constant, and concurrently supplying H₂, NH₃ andTMA.

About 3 μm in thickness of n-type GaN was formed on the buffer layer312, as an n-contact layer 313, under conditions controlled by keepingthe temperature of the sapphire substrate 311 at 1150° C. andconcurrently supplying H₂, NH₃, TMG and silane.

About 180 nm in thickness of non-doped In_(0.03)Ga_(0.97)N was formed onthe n-contact layer 313, as a strain relaxation layer 314A, underconditions controlled by lowering the temperature of the sapphiresubstrate 311 to 850° C., keeping the temperature constant andconcurrently supplying N₂ or H₂, NH₃, TMG and TMI.

After forming the strain relaxation layer 314A, about 200 nm inthickness of n-type Al_(0.05)Ga_(0.95)N was formed on the strainrelaxation layer 314A, as an n-cladding layer 314B, under conditionscontrolled by raising the temperature of the sapphire substrate 311 to1150° C., keeping the temperature constant and concurrently supplying N₂or H₂,NH₃, TMG, TMA and silane.

About 3.5 nm in thickness of Al_(0.13)Ga_(0.87)N was formed on then-cladding layer 314B, as a barrier layer 351, concurrently supplying N₂or H₂, NH₃, TMG and TMA. And about 3 nm in thickness ofIn_(0.05)Ga_(0.95)N was formed on the barrier layer 315, as a well layer352, concurrently supplying N₂ or H₂, NH₃, TMG and TMI. Similarly, fourpairs of the barrier layer 351 and the well layer 352 were formed insequence under the respective same conditions, and then a barrier layer351 made of Al_(0.13)Ga_(0.87)N was formed on the fifth pair of thebarrier layer 351 and the well layer 352. Accordingly, an emission layer315 having a multi-quantum well (MQW) structure was formed.

About 25 nm in thickness of Mg-doped p-type Al_(0.15)Ga_(0.85)N wasformed on the emission layer 315, as a p-cladding layer 316, underconditions controlled by keeping the temperature of the sapphiresubstrate 311 at 1150° C. and concurrently supplying N₂ or H₂, NH₃, TMG,TMA and CP₂Mg.

About 100 nm in thickness of Mg-doped p-type GaN was formed on thep-cladding layer 316, as a p-contact layer 317, under conditionscontrolled by keeping the temperature of the sapphire substrate 311 at1100° C. and concurrently supplying N₂ or H₂, NH₃, TMG, and CP₂Mg.

An etching mask is formed on the p-contact layer 317, and apredetermined region of the mask is removed. Then, exposed portions ofthe p-contact layer 317, the p-cladding layer 316, the emission layer315, the strain relaxation layer 314A, and some part of the n-contactlayer 313 were etched by a reactive ion etching using gas includingchlorine (Cl). Accordingly, the surface of the n-contact layer 313 wasexposed.

Then, an electrode 318B and an electrode 318A which transmits lightswere formed on the n-contact layer 313 and the p-contact layer 317,respectively, as follows.

(1) A photoresist layer was laminated on the n-contact layer 313. Awindow was formed on a fixed region of the exposed surface of then-contact layer 313 by patterning using photolithography. Afterexhausting in high vacuum lower than 10⁻⁴ Pa vacuum order, about 20 nmin thickness of vanadium (V) and about 1800 nm in thickness of aluminum(Al) were deposited on the window. Then, the photoresist layer laminatedon the n-contact layer 313 was removed. Accordingly, the electrode 218Bwas formed on the exposed surface of the n-contact layer 313.

(2) A photoresist layer was laminated on the p-contact layer 317. Thephotoresist layer of an electrode forming part on the p-contact layer317 was removed by patterning using photolithography, and a window wasformed there.

(3) After exhausting in high vacuum lower than 10⁻⁴ Pa vacuum order,about 1.5 nm in thickness of cobalt (Co) and about 6 nm in thickness ofgold (Au) were formed in sequence on the photoresist layer or theexposed surface of the p-contact layer 317 in a reaction chamber fordeposit.

(4) The sample was took out from the reaction chamber for deposit. Thencobalt (Co) and gold (Au) laminated on the photoresist layer wereremoved by a lift-off, and an electrode 318A which transmits lights isformed on the p-contact layer 317.

(5) To form an electrode pad 320 for a bonding, a window was formed on aphotoresist layer, which was laminated uniformly on the electrode 318A.About 1500 nm in thickness of cobalt (Co), nickel (Ni) or vanadium (V)and gold (Au), aluminum (Al) or an alloy including at least one of thosemetals were deposited on the photoresist layer. Then, as in the process(4), cobalt (Co), nickel (Ni) or vanadium (V) and gold (Au), aluminum(Al) or an alloy including at least one of those metals laminated on thephotoresist layer were removed by a lift-off, and an electrode pad 320was formed.

(6) After the atmosphere of the sample was exhausted by a vacuum pump,O₂ gas was supplied until the pressure becomes 3 Pa. Under conditionscontrolled by keeping the pressure constant and keeping the temperatureof the atmosphere about 550 ° C., the sample was heated for about 3 min.Accordingly, the p-contact layer 317 and the p-cladding layer 316 werechanged to have lower resistive p-type, and the p-contact layer 317 andthe electrode 318A, and the n-contact layer 313 and the electrode 318B,respectively, are alloyed.

Through the process of (1) to (6), the light-emitting device 300 wasformed.

Various samples of an n-cladding layer 314B made of n-typeAl_(0.05)Ga_(0.95)N , each having a different electron concentration,were formed in the same process described above. FIG. 8 illustrates theelectroluminescence (EL) luminous intensity of the light-emitting device300 having the n-cladding layer. As shown in FIG. 8, the luminousintensity of the light-emitting device 300 becomes larger around8×10¹⁷/cm³.

Various samples of an n-contact layer 313 made of n-GaN, each having adifferent electron concentration, were formed in the same processdescribed above. FIG. 9 illustrates the electroluminescence (EL)luminous intensity of the light-emitting device 300 having the n-contactlayer. As shown in FIG. 9, the luminous intensity of the light-emittingdevice 300 becomes larger in accordance that the electron concentrationof the n-contact layer becomes 1.1×10¹⁸/cm³, 8×10¹⁷/cm³, and 4×10¹⁷/cm³.

Because the hole concentration of each p-layers (the p-cladding layerand the p-contact layer) is 7×10¹⁷/cm³, the luminous efficiency becomelarger when a ratio of the electron concentration of n-layer to the holeconcentration of the p-layer is in the range of 0.5 to 2.0. In short,the value of dividing the electron concentration by the holeconcentration is in the range of 0.5 to 2.0. Preferably, the ratio ofthe hole concentration of the p-layer and the electron concentration ofn-layer should be in the range from 0.7 to 1.43, more preferably, from0.8 to 1.25.

In the third embodiment, the light-emitting device 300 has the emissionlayer 315 with multi quantum-well (MQW) structure. Alternatively, theemission layer 315 can have a single quantum-well structure.

In condition that the ratio of the electron concentration of the n-layerto the hole concentration of the p-layer is as in the above embodiment,the barrier layer, the well-layer, the n-cladding or the p-claddinglayer, and the n-contact or the p-contact layer can be made ofquaternary, ternary, or binary nitride compound semiconductor whichsatisfies the formula Al_(x)Ga_(1−x−y)In_(y)N, (0≦x≦1, 0≦y≦1), having anarbitrary composition ratio.

The luminous efficiency of the light-emitting device 300 may becomesmaller without an n-cladding layer or a strain relaxation layer, but itcan be larger than that of the conventional light-emitting device.

In the third embodiment, magnesium (Mg) was used as a p-type impurity.Alternatively, Group II elements such as beryllium (Be), zinc (Zn), etc.can be used.

The device in the present invention can be applied not only to alight-emitting device but also a light-receiving device.

(Other Embodiment)

The following methods can be applied to the above embodiments.

(1) A method for Forming a Buffer Layer

In the above embodiments, a buffer layer is formed at a low temperatureof 400° C. to 600° C. Alternatively, a buffer layer can be formed at atemperature of 1000° C. to 1180° C. by MOCVD. Preferably, thetemperature should be in the range of 1050° C. to 1170° C., and morepreferably, 1100° C. to 1150° C. A buffer layer made of AlN having athickness of 2.3 μm is formed on a sapphire substrate, at a growthtemperature of 1050° C., 1110° C., 1130° C., 1150° C., 1170° C., and1200° C. A GaN layer having a thickness of 2 μm is formed on the bufferlayer at the same growth temperature, and a surface mophology of the GaNlayer is observed by an optical microscope. The surface mophology of theGaN layer is best when the growth temperature of the buffer layer is1130° C. The surface mophology of the GaN layer better when the growthtemperature of the buffer layer is 1110° C. and 1150° C., less better1050° C., 1170° C. When the growth temperature of the buffer layer is1200° C., the surface mophology of the GaN is not good. Accordingly, thegrowth temperature of the buffer layer is preferably in the range of1000° C. to 1180° C. as described above.

In the above embodiment, a buffer layer is made of AlN. Alternatively, abuffer layer can be made of GaN, InN, Al_(x)Ga_(1−x)N (0<x<1),In_(x)Ga_(1−x)N (0<x<1), Al_(x)In_(1−x)N (0<x<1), andAl_(x)Ga_(y)In_(1−x−y)N (0<x<1, 0<y<1, 0<x+y<1). Alternatively, a bufferlayer can be also made of Al_(x)Ga_(y)In_(1−x−y)N (0≦x≦1, 0≦y≦1,0≦x+y≦1)in which a part of the group III element is changed to boron (B)or thallium (Tl), and a part of nitrogen (N) is changed to phosphorus(P), arsenic (As), antimony (Sb), bismuth (Bi), and so on. Furtheralternatively, a buffer layer can be doped with an n-type dopant such assilicon (Si), or a p-type dopant such as magnesium (Mg).

(2) After forming a nitride film having a thickness of 10 to 300 Å onthe sapphire substrate, a buffer layer made of AlN is formed. Athickness of the buffer layer should be preferably in the range of 1.2μm to 3.2 μm, and more preferably 1.5 μm to 3.3 μm. A growth rate of thebuffer layer should be preferably in the range of 10 nm/min. to 250nm/min. AlN layers, each having a thickness of 0.8 μm, 1.0 μm, 1.5 μm,2.3 μm, 3.0 μm, and 3.3 μm, was formed on the sapphire substrate at1130° C., at which temperature a surface mophology of a AlN layer is thebest. A GaN layer having a thickness of 2 μm was formed at 1130° C. onthe buffer layer in each of the samples, and a surface mophology of theGaN layer is observed.

When the thickness of the AlN layer is 2.3 μm, a perfect specularreflection can be obtained. When the thickness is 1.5 μm and 3.3 μm, anapproximate specular reflection can be obtained. But when the thicknessis 0.8 μm, 1.0 μm, and 3.0 μm, a specular reflection cannot be obtained,and a crystal growth is difficult on the GaN layer. Accordingly, thethickness of the buffer layer should be preferably in the rangedescribed above.

And after forming a nitrogenated layer having a thickness of 0 to 10 Åon the sapphire substrate, buffer layers made of AlN, each having athickness of 0.015 μm, 0.30 μm, 0.45 μm, 0.90 μm, 1.90 μm, and 2.30 μm,are formed at 1130° C. Then a GaN layer was formed on the buffer layerin each of the samples, and a surface mophology of the GaN layer isobserved.

When the thickness of the AlN buffer layer is 0.30 μm and 0.45 μm, aperfect specular reflection can be obtained. When the thickness is 0.015μm and 0.90 μm, an approximate specular reflection can be obtained. Butwhen the thickness is 1.90 μm and 2.30 μm, a specular reflection cannotbe obtained, and a crystal growth is difficult on the GaN layer.Accordingly, the thickness of the buffer layer should be preferably inthe range of 0.01 μm to 2.3 μm. The thickness should be preferably inthe range of 0.1 μm to 1.5 μm, more preferably 0.2 μm to 0.5 μm, themost preferably 0.3 μm to 0.45 μm.

(3) The sapphire substrate is desirably treated by a nitriding treatmentbefore forming a buffer layer. The sapphire substrate is cleaned underconditions controlled by raising the temperature in the chamber to 1000°C., keeping the temperature constant, and concurrently supplying H₂.Using H₂gas as a carrier, NH₃, hydrazine (H₂NNH₂), and/or organic amineare supplied to complete the nitriding treatment. The thickness of thenitride film on the sapphire substrate should be preferably in the rangeof 0 Å to 300 Å.

(4) In the other embodiment, a buffer layer is formed by MOCVD.Alternatively, MBE can be applied to form a buffer layer. Furtheralternatively, sputtering can be applied.

A buffer layer made of AlN can be formed by a reactive sputtering in aDC magnetron sputtering equipment, using a high purity metal aluminum(Al) and N₂gas as source materials. Alternatively, a buffer layer madeof Al_(x)Ga_(y)In_(1−x−y)N (0≦x≦1, 0≦y≦1, 0≦x+y≦1, where compositionratios x and y are arbitrary figures) using a metal aluminum (Al), ametal gallium (Ga), a metal indium (In), N₂ or NH₃ gas can be formed asin a step (1) above. As a method for forming the buffer layer,evapolating, ion plating, laser abration, and ECR can be applied tosputtering. These physical vapor deposit should be preferably carriedout at a temperature of 200° C. to 600° C., more preferably 300° C. to500° C., and further more preferably 400° C. to 500° C.

When using these physical vapor deposition, the thickness of the bufferlayer should be preferably in a range of 100 Å to 3000 Å. The thicknessshould be more preferably in a range of 100 Å to 2000 Å, and the mostpreferably 100 Å to 300 Å.

After the buffer layer is treated by a heat treatment in the atmosphereof H₂ and NH₃ gases for 5 minutes, a RHEED pattern was measured. As aresult, a crystallization of the buffer layer treated by a heattreatment is improved compared with that of the buffer layer which isnot treated by a heat treatment. A flow rate of H₂ gas and NH₃ gas usedin the heat treatment should be preferably 1:0.1 to 1:1. The flow rateshould be more preferably 1:0.1 to 1:0.5. A heating temperature shouldbe preferably in a range of 1000° C. to 1250° C., more preferably 1050°C. to 1200° C., and the most preferably 1100° C. to 1150° C. Varyingthese heating condition or the flow rate of gases, a RHEED pattern ofthe buffer layer was measured. As a result, a crystallization of thebuffer layer becomes better when a flow rate of gases and a heatingtemperature is in the range shown above. According to the result, asingle crystallization is considered to be improved by arecrystallization of the buffer layer.

A GaN layer having a thickness of 4 μm was formed on the buffer layer byMOCVD, and then a rocking curve of the GaN layer was measured by anX-ray diffraction equipment. As a result, a single crystallization ofGaN formed on the buffer layer, which is treated by the heat treatmentas described above, becomes even or better compared with that of GaNformed on a buffer layer which is formed on a substrate by using MOCVD.

Because the buffer layer is formed by a physical vapor deposit and heattreated at a high temperature, a single crystallization of the bufferlayer is promoted. As a result, the single crystallization of the GaNlayer is considered to be improved.

(5) When a well layer and a barrier layer grow at a temperature of 830°C. to 930° C. and a difference between growth temperatures of thebarrier and the well layers is Δ T≦50° C., a crystallization of anemission layer or an active layer is found to be improved. Here, thegrowth temperature of the barrier layer is higher than that of the welllayer.

(6) A substrate can be made of sapphire, spinel (MgAl₂O₂), silicon (Si),carbon silicide (SiC), zinc oxide (ZnO), gallium phosphide (GaP),gallium arsenide (GaAs), magnesium oxide (MgO), manganese oxide, etc.

A buffer layer in all the above embodiments can be formed not only at alow growth temperature but a high growth temperature. Also, a bufferlayer can be formed by sputtering.

While the invention has been described in connection with what arepresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A light-emitting device using gallium nitridecompound semiconductor comprising: an emission layer with a multiquantum-well (MQW) structure, in which a barrier layer and a well layerare formed alternately; wherein said barrier layer is made ofAl_(x)Ga_(1−x)N (0<x≦018).
 2. A light-emitting device using galliumnitride compound semiconductor according to claim 1, wherein said welllayer is made of In_(y)Ga_(1−y)N (0<y≦0.1).
 3. A light-emitting deviceusing gallium nitride compound semiconductor according to claim 1,wherein said barrier layer has a thickness from 2 nm to 10 nm.
 4. Alight-emitting device using gallium nitride compound semiconductoraccording to claim 1, wherein said barrier layer has a thickness from 3nm to 8 nm.
 5. A light-emitting device using gallium nitride compoundsemiconductor according to claim 1, wherein a luminous wavelength is inthe ultraviolet rays region.